Speech coding or speech compression relates to obtaining digital representation of a speech signal that can be used for digital transmission and storage of the speech signal. Typical speech coding schemes aim at storing or transmitting a speech signal with a minimal number of bits while maintaining the quality of the signal. An ideal coded speech signal has a low bit rate, high perceived quality, low complexity and low signal delay.
Speech coding methods can be broadly classified into waveform coding, vocoding and hybrid coding. Waveform coding is done by producing a reconstructed signal, whose waveform very much resembles the original speech waveform, without assuming any properties of the speech signal. This is done on a sample by sample basis. Various time domain waveform coding schemes are Pulse Code Modulation (PCM), APCM (Adaptive PCM), DPCM (Differential PCM), and ADPCM (Adaptive Differential PCM).
Vocoding is a general compression scheme for low bit rate speech coding and mainly depends on the voice generated. In vocoding, parameters of the vocal tract filter which are stored or transmitted to a decoder are extracted and the speech is then synthesized at the decoder using these parameters. Linear Predictive Coding (LPC), formant coders and phase vocoders are examples of vocoding. Another vocoding example is the U.S. government LPC algorithm (LPC-10) that is used for military applications operating at 2.4 Kbps.
Hybrid coding makes use of techniques used in both waveform coding and vocoding to achieve good quality speech at a reasonable bit rate. Hybrid coders, however, can be complex and computationally expensive. Hybrid coding uses a linear prediction source filter model of a speech production system. Typically, in the hybrid coding, analysis of a speech frame is done by synthesizing the same at the encoder to select an excitation signal by trying to minimize the error between the reconstructed speech waveform and the original speech waveform. Hybrid coders are broadly classified as Analysis by Synthesis (Abs) coders. Code Excited Linear Prediction (CELP) codecs, Algebraic CELP (ACELP), Conjugate Structure ACELP (CS-ACELP) codecs are examples of the hybrid codec category.